Between a young, muscular 25-year-old male and an older 75-year-old female with higher body fat percentage, which person is expected to have the higher percent body water and why?

• A. The older female because age-related loss of kidney concentrating ability increases extracellular water relative to intracellular compartments, which raises total body water percentage despite more fat
• B. The young muscular male because muscle contains more water than adipose tissue so higher lean mass increases total body water percentage
• C. They will have essentially identical percentages because sex and age do not meaningfully affect total body water when calorie intake is controlled
• D. The older female because hormonal changes after menopause cause increased water retention within adipose tissue leading to a higher overall water fraction when averaged across total body mass

Answer: (B)